1. Field of the Invention
The present invention relates to an electrophotographic image forming technique such as for printers, copiers, facsimile machines and the like. More particularly, the invention relates to an electrophotographic image forming technique adopting liquid development.
2. Description of the Related Art
Conventionally, the electrophotographic image forming apparatuses have been put to actual use, which are adapted to provide a predetermined image by taking the steps of: exposing a charged photosensitive member (image carrier) by means of exposure means thereby forming an electrostatic latent image on the photosensitive member; causing toner to adhere to the photosensitive member by means of developing means thereby developing the electrostatic latent image into a toner image; and transferring the toner image onto a transfer sheet. There have been known liquid development and powder development as a development system taken by the developing means. Liquid development has several advantages, which include: providing an image of higher resolution by virtue of the use of toner having a mean particle size of 0.1 to 2 μm, which is smaller than that of toner used in powder development; providing an image of a consistent quality because of the toner being provided as liquid developer having high fluidity; and the like. On this account, there have been proposed various types of image forming apparatuses using liquid development system (see, for example, Japanese Unexamined Patent Publication No. 7-209922 of 1995).
This conventional image forming apparatus includes a developing roller (liquid developer carrier) for transporting liquid developer toward a development position facing the photosensitive member while carrying the liquid developer on its surface, the liquid developer with charged toner dispersed in a carrier liquid. The charged toner in the liquid developer filling a gap (development gap) between the photosensitive member and the developing roller is transferred to the photosensitive member, thereby developing the electrostatic latent image on the photosensitive member into a toner image.
The image forming apparatus of liquid development system using liquid developer so arranged involves a problem that when an electric field applied to the charged toner at the development position varies or the toner density in the liquid developer varies, the density of the toner image formed by developing the electrostatic latent image varies. The electric field is affected by the variations of image forming conditions including a developing bias, exposure energy, charging bias and the like, and by the variations of a dimension of the development gap. Thus, the variations of the image forming conditions, the variations of the dimension of the development gap and the variations of the toner density in the liquid developer affect the density of the toner image, thus constituting causative factors of a degraded image quality of the toner image as exemplified by insufficient image density, image density variations and the like. In order to attain the image of a consistent quality, therefore, need exists for providing measure to prevent the density of the toner image from being affected by the variations of the image forming conditions or of the dimension of the development gap, or for controlling the toner density in the liquid developer with high accuracy.